As for an electric power tool in which a motor is used as an electric power source and a secondary battery is used as a power supply unit, there are provided various types of control drive circuits for driving the motor. (see, Japanese Patent Laid-open Application No. 2005-137087 (hereinafter, referred to as “Patent Reference 1”)).
In recently provided control drive circuits, mechanical contacts are removed from circuits between a power supply unit and a motor in order to improve vibration-resistance and durability, while an electric power is supplied to a control circuit in response to the ‘ON’ operation of a trigger switch and the supply of the power thereto is interrupted when a trigger switch is turned off in order to avoid the unnecessary consumption of a secondary battery.
An example of the above-described control drive circuit is shown in FIG. 3. In the drawing, reference symbol D denotes a battery pack in which a number of secondary battery cells is connected in series, reference symbol CPU denotes a control circuit configured with a single-chip microcomputer, and reference symbol SW1 denotes a trigger switch. Here, a brushless motor, in which coils are bridge-connected with each other, is used as a motor. Reference symbols U, V and W in FIG. 3 denote connection nodes to the coils of the motor. The electric power is supplied to the coils through upper switching elements F1, F3 and F5 of U, V and W-phases, respectively, and lower switching elements F2, F4 and F6 thereof, respectively. In the drawing, reference numeral 3 indicates a drive circuit for driving the upper switching elements, and reference numeral 4 indicates a drive circuit for driving the lower switching elements. In the drawing, reference symbols V•3, W•3, V•4 and W•4 denote connections between the control circuit CPU and drive circuits 3 and 4 of V and W-phase, respectively.
When the trigger switch SW1 is turned on, the electric power is supplied to the control circuit CPU. Next, according to the output signal of the control circuit CPU, when the switching element F1 is turned on by the operation of the drive circuit 3 for the U-phase upper switching element (the drive circuit 3 having, e.g., transistors Q6, Q7 and Q8), the switching element F4 is turned on by the operation of the drive circuit 4 for the V-phase lower switching element. Thereafter, when the switching element F3 is turned on by the operation of the drive circuit 3 for the V-phase upper switching element, the switching element F6 is turned on by the operation of the drive circuit 4 for the W-phase lower switching element. Furthermore, when switching element F5 is turned on by the operation of the drive circuit 3 for the W-phase upper switching element, the switching element F2 is turned on by the operation of the drive circuit 4 for the U-phase lower switching element, (the drive circuit 4 having e.g., transistors Q9, Q10 and Q11). In this manner, the three-phase motor is driven. Further, the electric power is supplied to each of the drive circuits 3 via a boost circuit 5. Moreover, diodes are connected in anti-parallel with the switching elements F 1 to F6 in order to brake the motor when stopping the motor by interrupting the supply of the electric power. To do so, transistors Q1 and Q2 for self-sustaining are provided such that the electric power is supplied to the control circuit CPU for a while after the trigger switch SW1 has been turned off.
In turning off the trigger switch SW1 to stop the operation of the motor, there arises no problem in case the supply of the electric power to the drive circuits 3 and 4 is interrupted prior to the control circuit CPU. However, in case the supply of the electric power to the control circuit CPU is interrupted first, the drive circuit 4 is turned on (the output of each of U-phase transistors Q11 and Q10 becomes high) when the output of the control circuit CPU is low, and thus an electric current flows through the switching element F2. At this time, the switching element F1, which is driven by the drive circuit 3 connected thereto via the boost circuit 5, is also turned on, so that a punch-through current flows through the bridge-structured switching element F1, therefore the switching elements are destroyed. The same problem occurs in the switching elements of other phases.